2009/03/11

Probably, this is the most complex photo that I have made to date, both because the number of exposures required, and because the processing required: to create the panorama which can be seen on the header of this blog, I had to take 27 different photographs, and then process them with a computer during several hours; however, I think the final result well deserves the effort.

The exposures

When preparing a panorama, after the tripod has been placed on the selected spot, the first think I usually do is to ensure that the camera is properly leveled; otherwise, strange deformations may appear during the stitching procedure. In cases like this one, the easiest way is to put the camera into an orientation such that the horizon gets at the middle of the viewfinder (I know, we are breaking the famous two-thirds rule, but that can be corrected later, just cropping the photograph). Then, turn the camera left and right, and check that the horizon stays at the middle of the viewfinder.

Experts recommend a 30% overlapping between images, but I prefer to play safe and do a 50% overlapping; to make this panorama and cover all the field of view that interested me, determined that a minimum of 3 positions was needed.

I wanted to use HDR techniques too, because the contrast between the lights and the shadows was too high. Then, for each camera position I made 9 exposures, ranging from one second to four minutes and sixteen seconds, all of them with the diaphragm fixed at f/10:

The panoramas

In theory, Hugin is capable of joinning an HDR panorama by itself; but in practice I was not getting a good result, and decided to find an alternate path: I created 9 panoramas, one for each exposure, and then made the HDR from these. Some people prefer to make an HDR for each camera position, and later join them in a panorama; but that method did not perform very well for me, especially when I tried to use href="http://wiki.panotools.org/Enfuse">Enfuse.

To make the nine panoramas, we could open with Hugin each set of three photographs separately, join them as if they where independent panoramas, and later make the HDR; but this method has two great dissadvantages:

On the darker exposures there are hardly a few visible spots, and it is difficult to make the images match.

There is no guarantee that the resulting panoramas will be aligned, because each one may have different control points.

In order to avoid these problems, what I did was calculate the panorama for the first set of three images, and then apply that same optimizations for the other groups:

First, I copied the files from the first set to IMG_1.tiff, IMG_2.tiff, IMG_3.tiff, and made a first panorama PANO_1.tiff with these.

Then, I overwrote IMG_1.tiff, IMG_2.tiff, IMG_3.tiff with the files from the second set, and made a second panorama PANO_2.tiff (avoiding to modify any parameter).

I repeated this procedure with the other sets of images.

The result is a set of nine perfectly aligned panoramas:

The processing

Now I joined those nine panoramas, using Enfuse and the default parameters; the result was good enought, and decided I did not need any tone-mapping. Using RawTherapee changed a bit the colours and the curves, reduced a bit of noise, and gave more sharpness; and finally with GIMP retouched some minor deffects.

The final result

The final result is a 6114 x 1378 pixels file, big enough to expand over half a meter, even if printed at high resolution: